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In an effort to find the best of the worst, scientists in the UK have pioneered a way to examine sperm using a process known as Raman spectroscopy.

The technology is geared to find sperm with healthy, intact DNA to ensure more successful in vitro fertilisation.

"It's interesting research because it shows that there are finer distinctions for sperm other than dead or alive," says Dr Michael Morris, a Raman spectroscopy expert at the University of Michigan who was not involved in the research.

During an average ejaculation somewhere between 200 and 500 million sperm are released. They twist their thread-like tails, known as flagella, to propel them forward in search of an egg to fertilise.

For infertile men, an average ejaculation might contain a low number of ill-formed and malfunctioning sperm, which dramatically reduces the likelihood that any of them will survive the harsh environment of a woman's reproductive tract to fuse with an egg.

For men with either low sperm counts or abnormal sperm, in vitro fertilisation offers a way around the problem.

Scientists use light microscopes to find a sperm cell, any sperm cell, and then inject it directly into an egg to fertilise it.

The problem with this, says Professor Alistair Elfick of the University of Edinburgh, who developed the technique using Raman spectroscopy, is that broken sperm often means broken DNA.

Popping the bonnet

DNA, not the sperm cell itself, is what is really important for a developing embryo.

"A sperm cell is pretty stripped down," says Elfick. "The DNA has no explicit role in delivering itself, it's just the payload."

In Raman spectroscopy scientists shine a beam of light on the 23 chromosomes encased in the head of the sperm.

Damaged DNA refracts light differently than intact DNA, so by examining the resulting light scientists can determine which DNA is the most likely to lead to a healthy human embryo. That's the theory at least.

Scientists haven't injected Raman spectroscopy-certified sperm into eggs and examined how many more embryos are created or how healthy they are.

Before the technique is used to create human life more testing is needed and federal approval will likely be necessary.

Limitations

But there are limits to the technique. Raman spectroscopy can only examine tens to hundreds of sperm cells at a time. For many infertile men this isn't a problem, since this is also roughly the number of sperm they can produce during an ejaculation.

Meanwhile, some scientists are skeptical about injecting Raman-examined sperm into an egg.

Elfick and Morris are confident that the energy contained in the laser beam is small enough not to cause any permanent damage to the sperm cell.